The dewatering of the fibrous material suspension applied to the wire cloth from above by filtration acquires major importance in papermaking processes. The fibrous material suspension is a mixture of suitable fibers, fillers, auxiliary chemical agents, and water which forms most of the mixture. In the paper industry, this filtration process is often also called sheet formation, and takes place in the wet or sheet forming part of the papermaking machine.
To be able to produce a paper sheet as uniform as possible, it is necessary to increase the proportion of water to on average 99% within the fibrous material suspension immediately before sheet formation. During the sheet forming process, this proportion is reduced to roughly 80% again by filtration. The paper fibers, fillers and auxiliary agents remain as fiber mat on the papermaking wire cloth.
While in the past dewatering took place mainly by papermaking wire cloth on Fourdrinier paper machines, double screen machines are being used more and more often today, preferably gap formers. They are also characterized in that the fibrous material suspension is sprayed directly into the gap between the two papermaking screens, and is dewatered by the two screens. With this type of papermaking machine, the filtration process can be accelerated such that production rates of 2000 meters/min and more are possible today.
One special field within the papermaking industry is the production of sanitary paper, such as Kleenex® tissues, toilet paper, paper towels, or the like. The type of paper being used here is characterized mainly by especially low G.S.M. between 10 and 20 g/m2 depending on the application. Graphic types of paper in comparison are between 42 and 120 g/m2.
To form a uniform sheet with such a low G.S.M., dilution of the fibrous material suspension, which is higher than for other types of paper, is required. The concentration of fibrous material drops to approximately 0.3 to 0.5%. To be able to also effectively produce these types of paper, this larger amount of water must take place in as short a time as possible, that is to say, at the highest production speeds. At the same time, retention of the fibrous material should remain as high as possible, that is to say, only a small portion of the added fibers should be removed with water.
In the prior art (EP 0 069 101 A1, EP 0 116 945 A1, EP 0 794 283 A1, and DE 100 30 650 A1), composite fabrics are known as papermaking wire cloths. Such wire cloths have two, more or less independent, single-layer wire cloths connected to each other in different ways. For the most part, they maintain an open surface so that the high required dewatering performance is thus guaranteed. The known solutions are aimed in most instances at suitably joining a uniform paper side in the form of a two-strand fabric, also called a basket weave, to the most varied machine sides in a suitable manner. Often, decreasing fibrous material retention favors high dewatering performance since the long wire bridges of the cross direction wires necessary for retention are not sufficiently available.
The joining of two single-layer fabrics into the papermaking wire cloth, in which longer wire bridges are formed by cross direction wires, is disclosed in EP 0 889 160 A1. The paper side (top) is implemented by a four-strand twill weave, and the machine side (bottom) is implemented by a four-strand basket weave. The two layers are joined by binding of a paper-side making direction wire to a machine-side cross direction wire. This type of fabric is characterized both by higher dewatering performance and by good fiber support based on the long wire bridges. In the known solution, currently unacceptable marking caused by the type of joining of the layers often occurs. The wear potential is limited, i.e., the machine side formed largely by the making direction wires is exposed directly to wear and, as a result, seam or wire cloth cracks can occur in use. The flexural stiffness in the transverse direction is limited due to the four-strand machine side and automatic seaming which is difficult to manage and which is caused by the lower making direction threads running parallel.